During ESD, large currents can flow through an Integrated Circuit (IC), potentially causing damage. To avoid this damage, protection circuits are added. One widely used ESD protection device is the Silicon Controlled Rectifier (SCR).
As typical SCR 100 as known in the art is illustrated in FIG. 1 exemplifying both a top view of and a cross section view along line A. The SCR 100 consists of two bipolar transistors, which are coupled such that during ESD operation they form a positive feedback loop. The first bipolar is a PNP, the bipolar emitter of which is the SCR anode 102, the bipolar base is known as the SCR trigger tap G2 104. The second bipolar is an NPN, the emitter of which is the SCR cathode 106, the bipolar base is the SCR trigger tap G1 108. The collector of the PNP and the base of the NPN is the same region. Also, the collector of the NPN and the base of the PNP is the same region.
One of the main advantages of the SCR is its high current handling capability. One of the main disadvantages of the SCR is its limited triggering speed, which might be too slow for very fast ESD events. The SCR can be used in a variety of configurations. In one configuration, it is triggered by the junction breakdown of the PNP base-collector junction, which is the same as the NPN collector-base region. In another configuration, the rising edge of the ESD event might trigger the SCR due to capacitive current flowing through both base-collector junctions.
The most important type of triggering, however, consists in applying a triggering signal at either the G1 trigger tap or the G2 trigger tap, or both. By increasing the voltage at the G1 tap, current will flow through the G1-cathode diode, which will trigger the SCR. Similarly, by decreasing the voltage at the G2 trigger tap, current will flow through the anode-G2 region, triggering the SCR.
Another widely used ESD device is a diode. In many ESD protection circuits a chain of diodes is used. However, during very fast ESD events, a voltage overshoot is associated with every diode. When placing N diodes in series, this overshoot is approximately multiplied by N. If this overshoot is significant, sensitive nodes in the circuitry (e.g. gate oxides) might be degraded or damaged during the ESD event.
Different implementations of diodes include gated diodes, STI diodes and Non-STI diodes. A top view and a cross section view along line B of a typical gated diode 200 is shown in FIG. 2. The diodes 200 include a N-well diode 202 of N-doped/N+ region 201 and a P-doped/P+ (anode) region 203 formed on a N-well region 204. The diodes 200 also include a P-well or P-substrate diode 206 having the N+ (cathode) region 205 and the P+ region 207 in a P-well or P-substrate 208. A local gate 210, such as a poly-gate, is placed between junction of the N+ 201 and P+ 203 and between the junction of N+ 205 and the P+ 207. This poly-gate 210 extends to a full width of the SCR. This gated diode 200 is fast, but adds more capacitance to the device due to the gated region extending through the full width of the diode. This helps in terms of reducing the overshoot voltage but at the same time adds more capacitance, which makes it difficult to use gated diodes in RF applications.
A typical STI diode 300 as shown in FIG. 3 is similar to the diode 200 except it does not include a poly-gate. The removal of the poly-gate provides a low capacitance to the device, but it has in most cases lower triggering speed, thus making these STI diodes less efficient for very fast ESD events. The STI diode 300 instead provides an isolation 302 between the junction of the N+ 201 and the P+ 203 and between the junction of the N+ 205 and the P+ 207. A typical Non-STI diode 400 as shown in FIG. 4 is similar to the STI diode 300 except it does not include the isolation 302 as described above. However this lack of isolation often leads to high leakage current.
Thus, there is a need in the art to provide an improved ESD protection device that overcomes the disadvantages of the prior art and provides for a fast triggering while reducing the overshoot voltage.